1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Background Art
For power semiconductor devices, semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors) and Diodes are publicly known each with a breakdown voltage of 600V, 1200 v or more. Such power semiconductor devices are used for electric power converters such as converters and inverters. In an electric power converter, an IGBT is used as a switching device and a diode is used for bypassing and freewheeling a current when the IGBT is turned-off.
An IGBT and a diode each formed as a discrete semiconductor component are generally combined to form a device. Recently, however, an RC-IGBT (Reverse Conducting IGBT) is proposed in which an IGBT and a FWD (Free Wheeling Diode) are formed on the same semiconductor substrate. FIG. 13 is a cross sectional view schematically showing a related RC-IGBT as a related semiconductor device. As is shown in FIG. 13, the related RC-IGBT has a configuration in which IGBT regions 100 and a diode region 110 are provided together on the same semiconductor substrate.
In the related RC-IGBT, on the top surface of the semiconductor substrate to be an n-drift region 101, an insulated gate of a MOS (Metal-Oxide-Semiconductor) structure is provided which is formed of a p+-base region 102, an emitter region 103, a gate insulator 104 and a gate electrode 105. A p+-base region provided in the diode region 110 is an anode region 112. The p+-base region 102 and the emitter region 103 have an emitter electrode 106 in contact therewith. The emitter electrode 106 is also in contact with the anode region 112 to function as an anode electrode.
On the bottom surface of the semiconductor substrate to be the n-drift region 101, the IGBT region 100 is provided with a p+-collector region 107 and the diode region 110 is provided with an n+-cathode region 117. The p+-collector region 107 is in contact with a collector electrode 108. The collector electrode 108 is in contact with the n+-cathode region 117 and functions as a collector electrode. By providing an IGBT and a diode in the same semiconductor substrate in this way, the device can be downsized and provided with a lowered cost compared with the case in which discrete components are used in combination.
For such an RC-IGBT, the following device is proposed. In the device, an emitter side structure is formed on the top principal surface side of a silicon substrate, an n-type buffer layer is formed on the bottom principal surface side, a p-type collector layer is formed in the principal surface of the n-type buffer layer, an n-type cathode region is selectively formed with a spacing apart from the p-type collector layer, a metal collector electrode is formed so as to be in contact with the p-type collector layer, a metal cathode electrode is formed so as to be in contact with the n-type cathode region and a part of the n-type buffer layer, and a diode is arranged as a current inhibiting device between the cathode electrode and a collector terminal (see JP-A-2000-200906, for example).
Moreover, for an RC-IGBT with lifetime control of carriers (hereinafter simply referred to as a lifetime control) in an n-drift region being carried out, the following device is proposed. The device is a reverse conducting semiconductor device having an insulated gate bipolar transistor and a commutating diode integrally formed in a substrate of a first conduction type semiconductor, in which device the commutating diode includes the second conduction type base layer and the first conduction type base layer of the insulated gate bipolar transistor, an emitter electrode on one surface of the substrate is made to be an anode electrode, a collector electrode on the other surface of the substrate is made to be a cathode electrode, and in apart of the first conduction type base layer, a short lifetime region is formed in which the lifetime of the carrier is shorter compared with the lifetimes of carriers in other first conduction type base layers (see JP-A-2005-317751, for example).
Furthermore, for another RC-IGBT with lifetime control being carried out, the following device is proposed. The device is a semiconductor device having an IGBT device region and a diode device region presented together in the same semiconductor substrate, in which device a short lifetime region shortening the lifetime of holes is formed in a region at least a part of a drift layer in the diode device region, which makes an averaged value of the lifetime of the holes in the drift layer including the short lifetime region is shorter in the diode device region than in the IGBT device region (see JP-A-2009-272550, for example).
In addition, for a method of lifetime control of an RC-IGBT, the following method is proposed. The method includes the material wafer preparing step, the crystal defect forming step and the mask layer removing step (see JP-A-2011-129619, for example).
In the material wafer preparing step, a material wafer is prepared which has an element forming layer the material of which is semiconductor, a mask layer provided on the bottom surface side of the element forming layer and having an opening section, and a boundary layer provided between the element forming layer and the mask layer and formed of a material different from the material of the element forming layer and the material of the mask layer.
In the crystal defect forming step, crystal defects are formed in the device forming layer by carrying out irradiation with charged particles from the bottom surface side of the mask layer.
In the mask layer removing step, the boundary layer is removed by carrying out etching using etching material reacting with the boundary layer and reacting with no element forming layer.
However, the optimum condition of an IGBT and that of a diode are different from each other. Therefore, there is a problem in that it is difficult to form an RC-IGBT with the optimum condition provided for each of the IGBT and the diode formed together in the same semiconductor substrate. The reason is that the ideal carrier concentration distribution in a turned-on operation of an IGBT and that of a diode formed together with the IGBT in the same semiconductor substrate are different from each other. The ideal carrier concentration distribution in a turned-on operation of the IGBT and that of the diode are as follows.
An IGBT is desirably made to have such a carrier concentration distribution that the carrier concentration on the emitter side becomes higher than the carrier concentration on the collector side at the turned-on operation. For that, in an IGBT, for example, the lifetime of the carriers on the emitter side is preferably longer than the lifetime of the carriers on the collector side. The reason is that the carrier concentration on the collector side made to be lower than that on the emitter side when an IGBT is turned-off can lower the concentration of residual carriers ejected when a depletion layer expands from the emitter side toward the collector side at switching to thereby lower the turn-off loss.
While, a diode is desirably made to have such a carrier concentration distribution that the carrier concentration on the anode side (the emitter side of the IGBT) becomes lower than the carrier concentration on the collector side (the collector side of the IGBT) at the turned-on operation. For that, in a diode, the lifetime of the carriers on the anode side is preferably shorter than the lifetime of the carriers on the cathode side. The reason is that the carrier concentration on the anode side made to be lower reduces the peak of a reverse current at the reverse recovery of the diode to permit soft recovery. This reduces noise at switching.
In each of JP-A-2005-317751, JP-A-2009-272550 and JP-A-2011-129619, it is described that an n-drift region is irradiated by light ions to locally form a region with short lifetime carriers to thereby carry out lifetime control. In JP-A-2005-317751, however, there is described that a region with short lifetime carriers is uniformly provided from an IGBT region to an FWD region in an n-drift region. Therefore, when the lifetime of the carriers on the anode side of the diode is shortened, the lifetime of the carriers on the emitter side of the IGBT is also shortened to result in an increase in the on-voltage of the IGBT. Moreover, when the lifetime of the carriers on the collector side of the IGBT is shortened, the lifetime of the carriers on the cathode side of the diode is also shortened to cause hard recovery at switching, which results in an increase of noise.
In JP-A-2009-272550, it is described that only the lifetime control of a diode is carried out. Therefore no lifetime control of an IGBT is carried out. This will cause difficulty in lowering a turn-off loss. In JP-A-2011-129619, it is described that the bottom surface of a semiconductor is irradiated by helium (He) ions to shorten the lifetime of the carriers on the anode side of a diode at the top surface side of the semiconductor substrate. This will cause the cathode side of the diode to be also irradiated by the helium ions to also result in shortened lifetime of the carriers on the cathode side of the diode. Therefore, it is impossible to carry out lifetime control under the optimum condition for the diode. In this way, with the methods described in JP-A-2005-317751, JP-A-2009-272550 and JP-A-2011-129619, it is impossible to carryout lifetime control under the optimum condition for each of the IGBT and the diode.
It is an object of the invention to provide a method of manufacturing a semiconductor device capable of improving switching characteristic thereof for solving the problems in the related device explained in the foregoing.